1. Field of the Invention
This invention relates to a transport guide member for transporting transfer sheets, and more particularly to a transport guide member for transporting toner-image-bearing transfer sheets to an image fixing unit with the back side thereof in contact with the transport guide member after a series of processes in which latent electrostatic images formed on a photoconductor in an electrophotographic copying apparatus are developed to visible toner images, the visible toner images are transferred to the surface of the above-mentioned transfer sheet, and the transfer sheet is separated from the photoconductor.
2. Discussion of Background
In an image formation apparatus in which an electrophotographic process is carried out in practice, such as an electrophotographic copying apparatus, a laser printer or a facsimile apparatus, a main charger 2, a light-image writing system 3, a development unit 4 and a transfer charger 5 are positioned around a photoconductor 1 in the sequence of the image formation processes along the rotational direction of the photoconductor 1, as shown in FIG. 1.
The outer surface of the photoconductor 1 is uniformly charged to a predetermined polarity by the main charger 2 and exposed to light images by using the light-image writing system 3, so that latent electrostatic images are formed on the photoconductor 1. The thus formed latent electrostatic images are developed to visible toner images using a developer supplied from a development unit 4. These toner images are transferred to a transfer sheet 6 by the aid of the transfer charger 5, and this transfer sheet is sent to an image-transfer section between the photoconductor 1 and the transfer charger 5. The transfer sheet 6 which bears the toner images is separated from the surface of the photoconductor 1 and transported to an image fixing unit 7, in which the toner images are fixed to the transfer sheet 6 and the image formation is finally achieved.
Japanese Laid-Open Patent Application 62-153865 discloses a transport guide member 8, as shown in FIG. 1, which is one of the means for transporting the toner-image-bearing transfer sheet 6 to the image fixing unit 7 after the transfer sheet 6 has been separated from the photoconductor 1. In this disclosure, the image-bearing transfer sheet 6 is carried to the image fixing unit 7 along the above-mentioned transport guide member, with the back side thereof in contact with the transport guide member.
In the course of an image-transfer operation, the transfer sheet is sufficiently charged with positive electric charges by the application of positive electric charges to the back side of the transfer sheet by a transfer charger, for example, a corona charger. The negatively charged toner images formed on the photoconductor are thus attracted to the positively charged transfer sheet, whereby the toner images are transferred to the transfer sheet.
When the transfer sheet employed for the image formation has a low resistivity, the positive electric charges which are imparted to the transfer sheet by the transfer charger are dissipated in the horizontal direction in the transfer sheet. Therefore, the quantity of the positive electrical charge which functions to attract the negatively charged toner is decreased, so that toner images formed on the photoconductor are not adequately transferred to the transfer sheet.
In the case where the resistivity of the transfer sheet is within an optimum range, the positive electric charges which are imparted to the back side of the transfer sheet by the transfer charger are moved toward the surface thereof. As a result, the negatively charged toner particles are electrostatically attracted to the positive electric charges on the surface of the transfer sheet, and the toner images are satisfactorily transferred to the transfer sheet.
When the transfer sheet has a high resistivity, positive electric charges provided by the transfer charger accumulate on the back side of the transfer sheet, without moving toward the surface thereof. Thus, the electrostatic attraction force between the toner particles and the transfer sheet remains small, and the transfer sheet is sent to the image-fixing unit, with the positive electric charges remaining on the back side thereof.
The above-mentioned problem of the residual electric charges remaining on the back side of the transfer sheet is particularly observed in a small-size or low-price electrophotographic copying apparatus in which a quenching system for the transfer sheet is not provided or in which a special unit such as a quenching needle is not provided.
When a large quantity of the positive electric charge remains on the back side of the transfer sheet, the positive electric charges are easily discharged at a small gap between the transfer sheet and the transport guide member. This interrupts the image formation, and causes a blurring of the toner images formed on the transfer sheet. In particular, when the diameter of the photoconductor employed in the electrophotographic apparatus is so small that the transfer sheet is separated therefrom with a large curvature, and there is no process for quenching the electric charge of the transfer sheet in the apparatus, the quantity of electric charge is increased and the toner images formed on the transfer sheet easily deteriorate.
The above-mentioned discharging of the positive electric charges at a small gap between the transfer sheet and the transport guide member takes place depending on (i) the quantity of the positive electric charge remaining on the back side of the transfer sheet, (ii) the potential of the surface of the transport guide member and (iii) the distance between the transfer sheet and the transport guide member. Although the relationship among the above-mentioned three factors has been explained by the Paschen's law, the distance between the transfer sheet and the transport guide member is usually too small to be controlled and cannot be made constant since the transfer sheet is transported in contact with the transport guide member.